1. Field of the Invention
The present invention relates to a plasma processing apparatus used for plasma treatments, and particularly to a plasma processing apparatus comprising a matching circuit corresponding to the impedance of a plasma processing chamber of the plasma processing apparatus.
2. Description of the Related Art
FIG. 4 illustrates a conventional single-excitation plasma processing apparatus for plasma treatments such as CVD (chemical vapor deposition), sputtering, dry etching, ashing, etc.
The plasma processing apparatus shown in FIG. 4 comprises the matching circuit 2C shown in further detail in FIG. 5, which is provided between a radio frequency generator 1 and a plasma excitation electrode 4. The matching circuit 2C is provided for achieving impedance matching between the radio frequency generator 1 and the plasma excitation electrode 4.
Radio frequency power from the radio frequency generator 1 is supplied through the matching circuit 2C to a supply plate 3 and then to the plasma excitation electrode 4. The matching circuit 2C is contained in a matching box 2 comprising a dielectric housing, whereas the plasma excitation electrode 4 and the supply plate 3 are covered with a chassis 21 comprising a conductor.
The plasma excitation electrode (cathode electrode) 4 includes protrusions 4a provided on the lower surface thereof, and a shower plate 5 having many holes 7 is provided in contact with the protrusions 4a of the plasma excitation electrode 4. Also, a space 6 is formed between the plasma excitation electrode 4 and the shower plate 5. A conductive gas inlet pipe 17 is connected to the space 6, and an insulator 17a, for insulating the plasma excitation electrode 4 from the gas supply source side, is inserted at an intermediate position of the gas inlet pipe 17.
The gas introduced from the gas inlet pipe 17 is supplied through the holes of the shower plate 5 to the inside of a chamber 60 formed by a chamber wall 10. In FIG. 4, reference numeral 9 denotes an insulator for insulating the chamber wall 10 from the plasma excitation electrode 4. The exhaust system for the plasma apparatus is not shown in the drawing.
A wafer susceptor (susceptor electrode 8) which also serves as a plasma excitation electrode is provided in the chamber 60, and supported by a shaft 13, a substrate 16 being mounted on the wafer susceptor 8.
The lower end of the shaft 13 is connected to the chamber bottom 10A through a bellows 11 so as to hermetically close the chamber 60.
The wafer susceptor 8 and the shaft 13 can be vertically moved by the bellows 11 so that the distance between the plasma excitation electrodes 4 and 8 can be controlled.
The wafer susceptor 8 is grounded for DC voltage, and has the same potential as the chamber wall 10 for DC voltage.
In the plasma processing apparatus, electric power at a frequency of about 40.68 MHz is generally supplied to generate a plasma between both electrodes 4 and 8 for plasma treatments such as CVD (chemical vapor deposition), sputtering, dry etching, and ashing.
The main components of the plasma processing apparatus, including the plasma processing chamber, are typically manufactured by an apparatus manufacturer whereas the matching circuit is typically manufactured by a separate entity, usually one specializing in the manufacture of electrical components or circuits.
As a result, a user of the plasma processing apparatus must use the matching circuit 2C to perform impedance matching between the plasma processing chamber and the radio frequency generator for each plasma treatment such as sputtering, dry etching, ashing, or the like.
The impedance (load impedance) of the plasma processing chamber includes an impedance Z0 before plasma generation and an impedance Z1 after plasma generation.
The impedance Z0 is determined to some extent in a designing process by the manufacturer of the chamber and can be precisely measured for that specific chamber. However, plasma processing apparatuses having identical dimensions cannot realistically be manufactured. Hence, the produced plasma processing chambers have different impedances due to variations of the chamber dimensions from the specified nominal sizes.
Furthermore, after the plasma is generated, the impedance Z1 varies with process parameters including the flow rate of gas used, the degree of vacuum in the plasma processing chamber, and the distance between the two electrodes 4 and 8. Thus, the impedance Z1 will differ from one plasma treatment to the next in the same plasma processing apparatus.
For example, in a dry etching apparatus, the impedance Z1 varies with the type of thin film material to be etched and etching conditions such as the etching rate and the shape of a portion to be etched. Also, in a film deposition apparatus, the impedance Z1 varies with the process gas used for forming a thin film and deposition conditions such as the deposition rate and the structure of the thin film.
With the conventional plasma apparatus, the user adjusts the output impedance of the matching circuit 2C to the impedance Z0 At the beginning of the plasma processing, and after plasma discharge is started, plasma discharge is stabilized according to the impedance Z1.
However, the characteristic impedance of the power supply system of the radio-frequency generator 1 is 50 Ω, while the impedance (Z0 and Z1 of the plasma processing chamber CN is less than 10 Ω. Thus, the large difference between both impedances must be compensated in the matching circuit.
FIG. 5 is a conceptual diagram showing the outline of the circuit configuration of the matching circuit 2C.
The radio frequency generator 1 supplies electric power for plasma discharge to the plasma processing chamber CN through a coaxial cable 1C and the matching circuit 2C.
For example, in the case where the coaxial cable 1C has a characteristic impedance of 50 Ω, the frequency of radio-frequency power used for plasma discharge is 40.68 MHz, and the impedance Z1 is “3.6 Ω+j1.4 Ω”, the passive elements in the matching circuit 2C may be selected to provide matching. In this case, in order to achieve matching between the characteristic impedance of the coaxial cable and the impedance of the plasma processing chamber, a tuning inductor LT is fixed at 150 nH, and thus the capacitance of a load capacitor CL is 281 pF, and the capacitance of a tuning capacitor CT is 146 pF.
The relationship between the parameter of each of the passive elements and the impedance Z1 to be matched is confirmed by a Smith chart as illustrated in FIG. 6. The Smith chart is normalized by the characteristic impedance 50 Ω of the power supply system.
In FIG. 6, point A represents the start point of the impedance to be matched where the characteristic impedance of the coaxial cable 1C is 50 Ω.
Then, the impedance to be matched is moved from point A to point B on an admittance chart by the load capacitor CL.
At point B, the impedance to be matched has a real part corresponding to “3.6 Ω”.
The impedance to be matched is further moved from point B to point C on an impedance chart by the tuning inductor LT.
Finally, the impedance to be matched is matched with the value of point D, i.e., the impedance Z1, by adjusting the capacitance of the tuning capacitor CT.
Assuming that the impedance Z1 varies by 50% (in the variation range of load) due to the conditions of the process, the real part of 3.6 Ω varies between 5.4 Ω and 1.8 Ω. In this case, the capacitance of the load capacitor CL must be controlled in the range of 405 pF for controlling the real part of the impedance from 3.6 Ω to 5.4 Ω to 225 pF for controlling the real part of the impedance from 3.6 Ω to 1.8 Ω with the center of the range at 275 pF.
Therefore, the capacitance of the load capacitor CL is controlled in the range of −20% (225 pF) to 44% (405 pF) with the center at 275 pF.
With the narrow control range, for example, the output impedance greatly varies with changes in the capacitance of the load capacitor CL. Namely, the control sensitivity of the output impedance to movement of a capacitance control increases. In other words, the output impedance is highly sensitive to changes in the load capacitor.
Thus, when the impedance is controlled by the matching circuit as shown comprising the variable passive elements such as the load capacitor CL and the like, the control sensitivity of the variable passive elements increases to cause problems in matching stability where a change in load (the impedance of the plasma processing chamber CN) occurs.
Even in the same processing apparatus the load varies according to a number of parameters. For example, the load changes with the lots and wafers to be processed due to the influence of the conditions of the process performed in the plasma processing chamber CN, the sizes and number of the wafers, and the reaction by-products produced in plasma processing.
Also, the plasma processing chamber CN may be opened for exchanging a part in the plasma processing chamber in a maintenance activity performed on the plasma processing chamber. This influences the dimensions of the apparatus, and thus changes the impedance of the plasma processing chamber CN after the maintenance work, causing the load change as a change in the plasma processing chamber Z1 after plasma discharge.
Furthermore, in a plasma processing system comprising a processing apparatus having a plurality of plasma processing chambers, or a plurality of plasma processing apparatuses, the impedance varies with the plasma processing chambers because the plasma processing chambers in the same apparatus are different in dimensions due to variations during the manufacture of the apparatus. Therefore, in this case, the load change occurs as an impedance change associated with the plasma processing chambers after plasma discharge.
Therefore, when impedance matching is performed by the matching circuit comprising the variable passive elements having high control sensitivity, matching to the load change is a sensitive process and hence difficult to control, even with only a small load change. Therefore, a conventional matching circuit has the problem of stability even in ordinary use.
As a method for solving the above problem, an amplifier for changing the impedance is contained in the matching circuit provided directly above the plasma processing chamber CN, in order to decrease the sensitivity of impedance control.
However, during use, the temperature of the plasma processing chamber is increased according to the conditions of the process. Thus, thermal stress corresponding to the increase in the temperature is applied to the matching circuit provided directly above the plasma processing chamber, and thus the characteristics of an active element constituting the amplifying circuit vary with the temperature. This thermal stress thus causes a deterioration in the stability of the circuit to impedance change.
In a conventional example, the difference between the input impedance of the matching circuit and the impedance of the plasma processing chamber is decreased, and the characteristic impedance of the coaxial cable for supplying electric power to the matching circuit from the radio frequency generator is controlled to less than 50 Ω which is close to the output impedance of the matching circuit. In this example, the control sensitivity of the matching circuit is decreased to decrease a variation in matching with the load change.
However, in this example, the characteristic impedance of the coaxial cable is normalized, and thus a plurality of special coaxial cables must be ordered from the manufacturer according to the impedance of the plasma processing chamber, which changes based on a plurality of parameters, thereby causing a problem in which an expensive nonstandard coaxial cable is used as the coaxial cable. Accordingly, it is desirable to provide impedance matching circuitry having low control sensitivity and greater stability regarding load changes.